Most boilers will have a max temperature of 90deg c. Assuming you can turn the temp control without the knob, turn it completely off and make a mark with nail varnish on the boiler and on the "knobless" shaft. Now turn it to maximum and make a mark on the boiler. Halfway between the two marks should be 40 deg c.
Order a new knob from Halsted or try and get a freebie.
Good luck
Paul
February 2009